Semiconductor laser with integrated phototransistor

ABSTRACT

The present invention relates to a semiconductor laser for use in an optical module for measuring distances and/or movements, using the self-mixing effect. The semiconductor laser comprises a layer structure including an active region ( 3 ) embedded between two layer sequences ( 1, 2 ) and further comprises a photodetector arranged to measure an intensity of an optical field resonating in said laser. The photodetector is a phototransistor composed of an emitter layer (e), a collector layer (c) and a base layer (b), each of which being a bulk layer and forming part of one of said layer sequences ( 1, 2 ). With the proposed semiconductor laser an optical module based on this laser can be manufactured more easily, at lower costs and in a smaller size than known modules.

FIELD OF THE INVENTION

The present invention relates to a semiconductor laser, in particular avertical cavity surface emitting laser (VCSEL), comprising a layerstructure including an active region embedded between two layersequences and comprising a photodetector arranged to measure theintensity of an optical field resonating in said laser. The inventionfurther relates to an optical sensor module for measuring distancesand/or movements and including such a semiconductor laser.

BACKGROUND OF THE INVENTION

An important application of the proposed semiconductor laser is the usein an optical sensor module measuring distances and/or movements. Suchan optical sensor module comprises at least one optical sensor includinga laser having a laser cavity for generating a measuring beam,converging means for converging the measuring beam in an action planeand for converging measuring beam radiation that is reflected by anobject into the laser cavity to generate a self-mixing effect and meansfor measuring the self-mixing effect, which means comprise aradiation-sensitive detector and associated signal-processing circuitry.Such an optical sensor module may be included in an optical input devicethat is based on the movement of an object and the device with respectto each other, but may also form part of measuring apparatuses ofdifferent types.

U.S. Pat. No. 6,707,027 discloses such an optical input device that usesthe self-mixing effect. Laser self-mixing occurs if an externalreflector, or object, is arranged in front of a laser so that anexternal laser cavity is obtained. In the case of an input device,movement of the device and the object, i.e. the reflector, which may bea human finger or a desk surface, with respect to each other causestuning of the external cavity. Such tuning results in re-adjustment ofthe laser equilibrium conditions and thus in detectable changes in thelaser output power. These changes, or undulations, are repetitive as afunction of the displacement of the object over a distance equal to halfthe wavelength of the laser radiation along the axis of the light beam.This means that the laser undulation frequency becomes proportional tothe speed of the external reflector. A measuring device based on laserself-mixing shows high sensitivity, and thus accuracy, which can beattributed to the fact that reflected laser radiation re-entering thelaser cavity determines the laser frequency and thus is amplified in thelaser cavity. In this way, high receiver sensitivity is obtained withoutthe use of additional means, like optical filters, or complex devicessuch as interferometers. An optical input device of this type equippedwith two diode lasers allows measurement of movements of the device andthe object with respect to each other in two mutually perpendicular (x-and y-) directions and any intermediate direction. Such a device can beused to navigate or move a cursor across a display panel, for example,to select an icon on the display.

U.S. Pat. No. 5,892,786 discloses an output control of a verticalmicrocavity light emitting device. This device includes a VCSEL-typediode laser embedded between two DBR stacks, wherein a phototransistoris embedded in one of the DBR stacks. With the output of thephototransistor, which measures the intensity of the optical fieldinside of the laser cavity, the output power can be controlled toachieve a constant level. The device of this document neither uses theself-mixing effect nor is it designed to convert measuring radiationfrom an object into a measuring signal. The heterojunctionphototransistor of this module comprises a layer which includes aquantum well. This quantum well increases the wavelength selectivity ofthe phototransistor to detect only radiation having the desired laserwavelength of stimulated emission and not the broad wavelength range ofspontaneous emission. Due to this quantum well, the gain of thephototransistor is large. Such a device, however, is not suitable forapplications such as measuring distances and/or movements with highaccuracy using the self-mixing effect.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a semiconductorlaser and an optical module for measuring distances and/or movements,using the self-mixing effect, in which optical module the semiconductorlaser can be used and, in addition, manufactured more easily, at lowercost and in a smaller size than in known modules.

The object is achieved with the semiconductor laser and the opticalsensor module according to claims 1 and 8. Advantageous embodiments ofthis semiconductor laser and the optical sensor module are subjectmatter of the dependent claims or are described in the portions of thedescription and preferred embodiments given hereinafter. The proposedsemiconductor laser comprises a layer structure including an activeregion embedded between two layer sequences and further comprises aphotodetector arranged to measure the intensity of an optical fieldresonating in said laser. The photodetector is a phototransistorcomposed of an emitter layer, a collector layer and a base layer, eachof which being a bulk layer and forming part of one of said layersequences, in the following also referred to as layer stacks.

The semiconductor laser is preferably a vertical cavity surface emittinglaser (VCSEL), with the two layer stacks forming the two end mirrors ofthe laser cavity. These end mirrors may be, for example, distributedBragg reflectors (DBR). Alternatively, the semiconductor laser may alsobe designed to form an edge-emitting laser, with the two layer sequencesforming the cladding layers for guiding the laser radiation within theactive region.

In the proposed semiconductor laser a phototransistor having an emitterlayer, a collector layer and a base layer, all of which are bulk layers,is embedded in one of the layer stacks or layer sequences. Bulk layersare understood to mean layers which do not include any additionalstructure, such as for example a quantum well structure. The activeregion is understood to mean the region or layer in which the laserradiation is generated.

The present invention is based on the insight that for an optical moduleused in a measuring device for measuring distances and/or movements,using the self-mixing effect, a quantum well as proposed in U.S. Pat.No. 5,892,786 should be avoided in order to achieve a high sensitivityand accuracy of the measuring device.

In the proposed semiconductor laser the photocurrent of thephototransistor should be large enough to overcome shot noise limitingthe signal to noise ratio of the system. The gain of the transistor onthe other hand should be low to avoid excessive phototransistor current.In other words, the base current of the phototransistor should be sohigh that the transistor has a low gain factor, preferably in the rangebetween 1 and 10, to avoid excessive collector current. Therefore, thebase, emitter and collector layers are preferably arranged close to theactive region or layer of the semiconductor laser. In a preferredembodiment, the phototransistor is arranged in the portion of the layerstack in which the intensity of the optical field still exceeds at least10% of the peak intensity of the VCSEL, which is achieved in thevicinity of the active region, although 3% can also be sufficient. Bymeans of this measure, the ratio between stimulated emission andspontaneous emission is substantially increased, so that wavelengthselectivity becomes a minor issue. Furthermore, the photon intensitycloser to the active region is high enough to provide a sufficientlyhigh photocurrent to overcome the shot noise limit. In an alternativeembodiment, the phototransistor is placed in the half of the layersequence which is located closer to the active region than the otherhalf. Preferably, the phototransistor is arranged in the upper half ofthe n-side DBR, that is that part of the n-side DBR that is closer tothe active layer.

When the proposed semiconductor laser is designed to form a VCSEL, thelayers of the phototransistor are formed of layers which are alreadyexistent in one of the layer stacks forming the end mirrors of the lasercavity. These layers are doped only to have an appropriate bandgap toform the base, collector and emitter layer of the phototransistor.Preferably the collector layer is formed of a high bandgap material andthe base layer and the emitter layer are formed of a low bandgapmaterial. A low bandgap material is a material which has a bandgap whichis lower than the photon energy of the optical field resonating in thelaser cavity. On the other hand, the high bandgap material has a bandgaplarger than the photon energy, typically significantly larger. For thecomplete base and emitter region, preferably the layer thickness isselected equal to one quarter optical wavelength and the layer positionis such that the base resides at an optical field intensity peak and theemitter is at a null of the optical field.

Such a semiconductor laser with the proposed structure of thephototransistor can be manufactured at low cost, since only bulk layersare used. Furthermore, unlike such a semiconductor laser without aphototransistor, no additional layers have to be deposited. Theinvention uses layers which are already included in the layer stacks ofsuch a laser. By adapting the bandgap of the material of the relevantlayers of the corresponding layer stack, these layers can be configuredto form the phototransistor, so that no additional layers are needed, asalready mentioned above, and the manufacture of the module becomeseasier. Usually the layer stacks, in which the phototransistor isembedded, are layer stacks of distributed Bragg reflectors. By using aphototransistor instead of a photodiode, advantageous use can be made ofthe amplification of the transistor, so that an improved signal comparedto a photodiode is obtained. Furthermore, no extra contact is requiredto the p-layer, which would form the anode of the photodiode and, in thecase of a phototransistor, would form the base.

In an embodiment of the semiconductor laser, the emitter layer of thephototransistor is set to ground potential. This provides a groundcontact for the laser current and a ground terminal for thephototransistor, such that both the laser anode and the phototransistorcollector can be driven at a positive voltage with respect to ground,thus simplifying the powering scheme.

In a further embodiment, the semiconductor laser is designed to be a topemitting diode laser and the phototransistor is embedded in the layersequence forming an n type reflector arranged opposite the main emittingside of the top emitting diode laser. Also in this embodiment, thecollector region is preferably made in a high bandgap material and thebase and emitter regions are both made in a low bandgap material, i.e.low enough to allow photon absorption to take place.

The proposed optical module comprises at least one such semiconductorlaser emitting a measuring beam which, when reflected by an object,re-enters the laser cavity and generates a self-mixing effect which ismeasured by the phototransistor. Such an optical measuring module formeasuring distances and/or movements also includes or is connected withan appropriate signal-processing circuitry which calculates the distanceand/or movement, based on the measuring signal of the phototransistor.Such an optical module may be embedded in an input device or in anapparatus in which such an input device is included, since the sensormodule according to the invention allows reducing the size and cost ofthe input device and thereby enlarges the field of applications.Including such an input device in an apparatus not only saves costs andspace, but also provides the designer with more freedom of design. Theinput device wherein the invention is implemented may have the sameconstruction as the laser self-mixing devices described in U.S. Pat. No.6,707,206 (which is incorporated herein by reference), with theexception of the integrated VCSEL and phototransistor structure.Apparatuses wherein the input device can be used are for example a mousefor a desktop computer, a notebook computer, a mobile phone, a personaldigital assistant (PDA) and a handheld game computer. The invention canalso be used in professional measuring apparatuses for measuring, forexample, distance to an object or movement of the object, movement of aliquid and movement of particles embedded in a liquid. Generally theinvention may be used in any applications wherein the laser self-mixingeffect can be used.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described herein after.

BRIEF DESCRIPTION OF THE DRAWINGS

The proposed semiconductor laser is described in the following by way ofexamples and with reference to the accompanying figures, withoutlimiting the scope of protection as defined by the claims. The figuresshow:

FIG. 1 a typical setup of a VCSEL laser as can be used in the presentinvention;

FIG. 2 a schematical view showing the location of the phototransistorrelative to the optical field in the n side mirror of a VCSEL accordingto an example of the invention; and

FIG. 3 a cross-section of an example of an input device.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows a schematic view of the basic design of a VCSEL as it canbe used in the present invention. The VCSEL comprises a lower layerstack 1 and an upper layer stack 2 with an active layer 3 embedded inbetween. This VCSEL is formed on a substrate 4 which may also comprise acooling function. The lower and upper layer stacks are typicallydesigned to form p- and n-type DBRs. The two DBRs are the end mirrors ofthe laser cavity, wherein the upper p-type DBR is formed so as to bepartly transmissive, for example with a reflectivity of 98% for thegenerated laser radiation, to function as an outcoupling mirror. Theupper p-type DBR and lower n-type DBRs are typically composed ofalternating layers of high and low refractive index GaAs (high index)and AlAs (low index) layers. The GaAs layers have a low Al % such thatthe material bandgap is above the photon energy.

In the present example, the phototransistor is embedded in the lowerlayer stack 1, which is the n side mirror (n-type DBR) of the VCSEL.FIG. 2 shows the location of the phototransistor relative to the opticalfield in this mirror.

A thin p-doped layer is placed at a GaAs/AlAs junction in the n-dopedmirror of the VCSEL to form the base of the phototransistor. The upperline of FIG. 2 represents the material refractive index that is high inAlGaAs containing a low % of Al, quarter wavelength thick layers thatare low in AlGaAs layers containing a high % of Al (or AlAs). Thephototransistor layer is made of GaAs such that photons can createelectron hole pairs. The electrons diffuse out of the base b, mainlyinto the emitter e, which is also formed in the GaAs material. The totalthickness of the quarter wavelength thick GaAs layer is about 60 nm, 30nm of which are p-doped adjacent to the AlAs n-doped collector layer c.The emitter e has the same bandgap as the base b to prevent an increasein current gain, due to the band edge step, that would occur if theemitter e had a higher bandgap than the base. FIG. 2 also schematicallyindicates the conduction and valance band energies (Ec and Evrespectively). These represent un-doped material properties, withoutapplication of field.

The optical intensity profile is shown in the lower portion of FIG. 2.As can be seen from this figure, the optical intensity is at a null inthe emitter GaAs layer such that it does not contribute much to photonabsorption, even though it has a bandgap small enough for absorption. Onthe other hand, the base region is at an optical field peak and hassignificant photon absorption. The proper placement of the base regionnext to an AlAs collector simplifies the requirement of a low bandgapemitter (a higher bandgap in the emitter would increase the transistorgain) without causing too high, unnecessary optical losses and allows aneasy design of a low gain phototransistor.

In this embodiment, a low-gain integrated phototransistor has beendesigned by doping one mirror layer in a standard VCSEL differently andby a slight reduction of the Al % in that mirror layer such that it willdetect photons. This forms an inefficient detector that is preferablysituated in the mirror stack at a location which is close to the activelayer where the laser photon density is far higher than at a locationexternal to the laser. As a result, the intended photocurrent iscomparable to that of a VCSEL with an integrated photodiode but thedetection of unwanted spontaneous emission is very inefficient. Thephototransistor shares the emitter n-contact with the n-contact of thelaser, a p-type contact is not required. The substrate delivers thephotocurrent from the collector with the same polarity as the VCSEL.With a non-alloyed n-type contact, both the VCSEL and thephototransistor could be contacted without any additional layerthickness beyond that of a standard VCSEL.

FIG. 3 is a diagrammatic cross-sectional view of an example of an inputdevice according to the present invention. The device comprises, at itslower side, a base plate 5, which is a carrier for the semiconductorlasers, in this embodiment the above-mentioned VCSEL-type lasers withintegrated phototransistor. In FIG. 3 only one laser 6 is visible, butusually at least a second laser is provided on the base plate 5 to beable to detect movements in two perpendicular directions. The lasersemit laser beams 9. At its upper side, the device is provided with atransparent window 8 across which an object 10, for example, a humanfinger is to be moved. A lens 7, for example a plano-convex lens, isarranged between the diode laser 6 and the window. This lens focuses thelaser beam 9 at or near the upper side of the transparent window. If anobject is present at this position, it scatters the beam 9. A part ofthe radiation of beam 9 is scattered in the direction of the laser 6.This part is converged by the lens 7 on the emitting surface of thelaser 6 and re-enters the cavity of this laser. The radiation returninginto the cavity induces changes in this cavity, which results in, interalia, a change of the intensity of the laser radiation emitted by thelaser 6. This change can be detected by the phototransistor of thelaser, which converts the radiation variation into an electric signal,and applies the electric signal to an electronic circuit 11 forprocessing this signal.

While the invention has been illustrated and described in detail in thedrawings and the foregoing description, such an illustration anddescription are to be considered illustrative or exemplary and notrestrictive, i.e. the invention is not limited to the disclosedembodiments. The different embodiments described above and in the claimscan also be combined. Other variations to the disclosed embodiments canbe understood and effected by those skilled in the art in practicing theclaimed invention, from a study of the drawings, the disclosure and theappended claims. For example, the VCSEL used can also be composed ofother material layers or may be a bottom emitting laser as known in theart. Furthermore, the semiconductor laser may also be designed as avertical extended cavity surface emitting laser (VECSEL). The number oflayers in the layer stacks is not limited by the present invention. Thisnumber can be selected appropriately for the required optical propertiesof the layer stack.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. The mere fact that measures are recited in mutually differentdependent claims does not indicate that a combination of these measurescannot be used to advantage. The reference signs in the claims shouldnot be construed as limiting the scope of these claims.

LIST OF REFERENCE SIGNS

-   1 lower layer sequence-   2 upper layer sequence-   3 active region-   4 substrate-   5 base plate-   6 laser-   7 lens-   8 transparent window-   9 laser beam-   10 object-   11 electronic circuit-   e emitter-   b base-   c collector

1. A vertical cavity surface emitting laser, comprising a layerstructure including an active region embedded between two layersequences and a photodetector arranged to measure an intensity of anoptical field resonating in said laser, wherein the photodetector is aphototransistor comprising an emitter layer (e), a collector layer (c)and a base layer (b), each of which being a bulk layer and forming partof one of said layer sequences (1, 2), the phototransistor beingarranged in a portion of the layer sequence in which the intensity ofthe optical field still exceeds 10% of a peak intensity which isachieved in the vicinity of the active region.
 2. (canceled)
 3. Thevertical cavity surface emitting laser according to claim 1, wherein thephototransistor is arranged in one of two halves of the layer sequencewhich half is located closer to the active region
 1. 4. The verticalcavity surface emitting laser device according to claim 1, wherein thetwo layer sequences are designed to form end mirrors of the laser, andthe emitter layer (e), the collector layer (c) and the base layer (b)are formed by appropriately adapting bandgaps of layers of the layersequences.
 5. The vertical cavity surface emitting laser according toclaim 4, wherein the collector layer (c) is formed of a high bandgapmaterial and the base layer (b) and the emitter layer (e) are formed ofa low bandgap material having a bandgap which is lower than the photonenergy of the optical field resonating in the laser.
 6. The verticalcavity surface emitting laser according to claim 1, wherein the emitterlayer is set to ground potential.
 7. The vertical cavity surfaceemitting laser according to claim 1, wherein the thickness of a regionformed by the base (b) and emitter layers (e) is equal to one quarter ofan optical wavelength of the optical field and said region is arrangedsuch that the base layer (b) is in an optical field intensity peak andthe emitter layer (e) is at a null of the optical field.
 8. Opticalsensor module for measuring distances and/or movements including atleast one vertical cavity surface emitting laser according to claim 1emitting a measuring beam which, when reflected by an object, re-entersthe laser cavity and generates a self-mixing effect which is measured bysaid phototransistor. 9-10. (canceled)